The mammalian target of Rapamycin, mTOR, is a cell-signaling protein that regulates the response of tumor cells to nutrients and growth factors, as well as controlling tumor blood supply through effects on Vascular Endothelial Growth Factor, VEGF. Inhibitors of mTOR starve cancer cells and shrink tumors by inhibiting the effect of mTOR. There are two important effects as mTOR inhibitors bind to the mTOR kinase. First, mTOR is a downstream mediator of the PI3K/Akt pathway. The PI3K/Akt pathway is thought to be over-activated in numerous cancers and may account for the widespread response from various cancers to mTOR inhibitors. The over-activation of the upstream pathway would normally cause mTOR kinase to be over-activated as well. However, in the presence of mTOR inhibitors, this process is blocked. The blocking effect prevents mTOR from signaling to downstream pathways that control cell growth. Over-activation of the PI3K/Akt kinase pathway is frequently associated with mutations in the PTEN gene, which is common in many cancers and may help predict what tumors will respond to mTOR inhibitors. The second major effect of mTOR inhibition is antiangiogenesis via the lowering of VEGF levels. These anticancer drugs have shown exceptional promise in cancer therapy and may change the way many types of cancer are treated.
Several studies have demonstrated that mTOR has a central role in controlling cell growth, proliferation and metabolism. mTOR regulates a wide range of cellular functions, including translation, transcription, mRNA turnover, protein stability, actin cytoskeletal organization and autophagy. mTOR is a member of the phosphoinositide kinase-related kinase (PIKK) family, but is not a phosphorylating phosphoinositide, a phosphorylate protein on serine or a threonine residue. There are two mTOR complexes in mammalian cells. mTOR complex I (mTORC1) is a raptor-mTOR complex, which mainly regulates cell growth in a rapamycin-sensitive manner, whereas mTOR complex II (mTORC2) is a rictor-mTOR complex, which regulates cytoskeletal organization in a rapamycin-insensitive manner.
Kinase subunits of both mTORC1 and mTORC2 regulate cell growth and survival in response to nutrient and hormonal signals. mTORC1 is activated in response to growth factors or amino-acids. Amino-acid-signaling to mTORC1 is mediated by Rag GTPases, which cause amino-acid-induced relocalization of mTOR within the endomembrane system. Growth-factor-stimulated mTORC1 activation involves AKT1-mediated phosphorylation of TSC1-TSC2, which leads to the activation of the Rheb GTPase that potently activates the protein kinase activity of mTORC1. Activated mTORC1 up-regulates protein synthesis by phosphorylating key regulators of mRNA translation and ribosome synthesis. mTORC1 phosphorylates eIF4EBP1 and releases it from inhibiting the elongation initiation factor 4E (eIF4E). mTORC1 phosphorylates and activates S6K1 at Thr-421, which then promotes protein synthesis by phosphorylating PDCD4 and targeting it for degradation. mTORC2 is also activated by growth factors, but seems to be nutrient-insensitive. mTORC2 seems to function upstream of Rho GTPases to regulate the actin cytoskeleton, probably by activating one or more Rho-type guanine nucleotide exchange factors. mTORC2 promotes the serum-induced formation of stress-fibers or F-actin. mTORC2 plays a critical role in AKT1 Ser-473 phosphorylation, which may facilitate the phosphorylation of the activation loop of AKT1 on Thr-308 by PDK1, which is a prerequisite for full activation. mTORC2 regulates the phosphorylation of SGK1 at Ser-422. mTORC2 also modulates the phosphorylation of PRKCA on Ser-657.
With the recent discovery of rapamycin independent function of mTOR (by mTOR2) in phosphorylation AKT (at S473), which is important in regulation of cell survival and modulation of PKCα, which plays a major role in regulation of actin cytoskeletal organization, it is believed that inhibition of mTOR function by rapamycin is partial. Therefore, a small molecule designed to compete with ATP in the catalytic site of mTOR would be expected to inhibit all of the kinase-dependent functions of mTORC1 and mTORC2, unlike rapalogs that only target mTORC1. Here we describe the discovery of direct mTOR kinase inhibitors which can be used in the treatment of a variety of cancers, including breast, lung, kidney, prostate, blood, liver, and ovarian cancers, and lymphoma and other indications such as rheumatoid arthritis, hamartoma syndromes, transplant rejection, multiple sclerosis and immunosuppression.
Phosphatidylinositol (hereinafter abbreviated as “PI”) is one of the phospholipids in cell membranes. In recent years it has become clear that PI also plays an important role in intracellular signal transduction. In particular, it is well recognized in the art that PI (4,5) bisphosphate (PI(4,5)P2) is degraded into diacylglycerol and inositol (1,4,5) triphosphate by phospholipase C to induce activation of protein kinase C and intracellular calcium mobilization, respectively [M. J. Berridge et al., Nature, 312, 315 (1984); Y. Nishizuka, Science, 225, 1365 (1984)].
PI3K was originally considered to be a single enzyme, but it has now been clarified that a plurality of subtypes is present in PI3K. Each subtype has its own mechanism for regulating activity. The PI3K family comprises at least 15 different enzymes sub-classified by structural homology and divided into 3 classes based on sequence homology and the product formed by enzyme catalysis. The class I PI3 kinases are composed of 2 subunits: a 110 kd catalytic subunit and an 85 kd regulatory subunit. The regulatory subunits contain SH2 domains and bind to tyrosine residues phosphorylated by growth factor receptors with a tyrosine kinase activity or oncogene products, thereby inducing the PI3K activity of the p110α catalytic subunit which phosphorylates its lipid substrate. Class I PI3Ks are further divided into two groups, class Ia and class Ib, in terms of their activation mechanism. Class Ia PI3Ks include PI3K p110α, p110β and p110δ subtypes, which transmit signals from tyrosine kinase-coupled receptors. Class Ib PI3K includes a p110γ subtype activated by a G protein-coupled receptor. PI and PI(4)P are known as substrates for class II PI3Ks. Class II PI3Ks include PI3K C2α, C2β and C2γ subtypes, which are characterized by containing C2 domains at the C terminus. The substrate for class III PI3Ks is PI only.
Most if not all of the non-rapalog mTOR inhibitors described to date in the scientific literature were developed to inhibit other enzymes, especially class I PI3Ks. Because PI3K regulates mTOR activity, inhibitors that target both enzymes are generally not useful as research tools to study mTOR regulation or function. However, drugs that are dual PI3K/mTOR inhibitors might have a therapeutic advantage over single-target inhibitors in certain disease settings. PI3K inhibitors and mTOR inhibitors are expected to be novel types of medicaments useful against cell proliferation disorders, especially as carcinostatic agents. Thus, it would be advantageous to have new mTOR inhibitors and PI3K inhibitors as potential treatment regimens for mTOR kinase- and PI3K kinase-related diseases.